g.setStroke( PARTICLE_VELOCITY_STROKE);
g.setColor( PARTICLE_VELOCITY_COLOR );
double[] a_ = new double[2];
Grid grid = env_.getGrid();
VelocityInterpolator interpolator = new VelocityInterpolator(grid);
double maxY = getMaxY();
for (Particle p : env_.getParticles()) {
if (options.getShowVelocities()) {
Vector2d vel = interpolator.findVelocity(p);
p.get(a_);
double x = (scale_ * a_[0]) + OFFSET;
double xLen = x + VELOCITY_SCALE * vel.x;
double y = maxY - scale_ * a_[1];